What could cause a low knocking sound in your engine after water has gone into the intake?

Answer 1

First, you would have to be sure that the knocking sound is due to the water. Water in the intake often results in 'hydrostatic lock' of an engine; that is, the water fills the cylinder(s) and makes it impossible for the pistons to move. This result in damage to throw rods/lifters or valves which could cause noise. If your engine is running, you aren't locked per se but you can make sure there is no water in there by removing your spark plugs and turning the starter over several times. This will give the cylinders a chance to eliminate the water by pressurizing the cylinder and shooting the water out of the spark plug holes. Then you can spray pressurized air into the holes to dry them out, check your plugs, take the opportunity to clean and gap the old plugs or put in new plugs, replace and start the car. If the noise persists you may need to remove the valve head cover and inspect your valves - you may also be able to visually inspect your throw rods or lifters. Make sure you have an appropriate gasket sealer on hand (I use RTV sealer) to replace the cover once done - you may also have to replace the gasket if it is damaged when you remove it. I strongly suggest you buy the appropriate Chilton's or other guide for your specific vehicle before you start taking stuff apart. If you feel at all less than fully confident in your mechanic skills, consider a professional mechanic.